Simply modifying a field will not normally change the current found set. However there are several script steps that could change the found set.
Is it the modification of one specific field that causes the change in the found set? Is it possible there is a script trigger attached to that field which is running a new Find (or using a Constrain step, or others)?
I will get the particulars of what the user is doing and report back. Thank you for your help. I am glad to hear that normally modifying a field from a record in a found set doesn't lose the found records. I'll get more detail.